This invention relates in general to fiber optic communications, and more particularly to optical receivers for use in fiber optic communication systems.
Many communications systems, such as those used to carry cable television signals, typically include a headend section for receiving satellite signals and demodulating the signals to an intermediate frequency (xe2x80x9cIFxe2x80x9d) or a baseband signal. The baseband or IF signal is then modulated with radio frequency (xe2x80x9cRFxe2x80x9d) carriers, then combined and converted to an optical signal for transmission from the headend section over fiber optic cable. Optical transmitters are distributed throughout the cable system for splitting and transmitting optical signals, and optical receivers are provided for receiving the optical signals and converting them to RF signals that are further transmitted along branches of the system over coaxial cable rather than fiber optic cable. Various additional devices are disposed in the television system to provide various functions. For example, devices known as taps are situated along the coaxial cable to split off the cable signal directed to the cable system subscribers. Amplifiers and hubs are disposed in the fiber optic system to receive, modify and boost the optical signal for further transmission over the fiber optic cable.
While cable systems have traditionally been designed in order to be one-way systems, that is for information to flow from the cable headend to the subscriber""s location, changes in the cable industry have necessitated the ability for information generated at subscriber locations to flow back to the headend. Accordingly, cable systems have recently modified the installed cable plant so as to have not simply a forward path, i.e., information flowing from the headend to the subscriber, but now to include a reverse path to allow information from the subscriber to flow back to the headend. Examples of information that would flow in the reverse path would include data relating to status monitoring of the subscriber device, subscriber payper-view program selections, cable modem information, and two-way video and telephony services. The need for information flowing from the subscriber back to the headend is anticipated to increase as cable television systems continue to add two-way interactive services, such as e-mail and web browsing.
In order to facilitate the easy flow of information in the forward and reverse paths, the cable system has divided the available spectrum into forward path and reverse path portions. Accordingly, information transmitted from the headend to the subscriber is typically in the frequency range of between approximately 50 megahertz (xe2x80x9cMHzxe2x80x9d) and 750 MHz. Conversely, information transmitted in the reverse path is typically in the frequency range from between about 5 MHz and 40 MHz. Various factors influence the ability to accurately transmit and receive optical signals within a cable television system. As the length of fiber optic cable within a system increases, for example, signal losses also increase. Further, temperature fluctuations which cause variation in the optical modulation index of the optical transmitter can result in variation of the RF output level of the optical receiver. Signal distortions may also be caused by non-linearities in the optical transmitter laser and photo diode of the optical receiver. Finally, many of the devices interposed in the forward and reverse paths themselves introduce noise and other distortions into the system. Accordingly, in many instances the range of the particular system in question is limited both in terms of distance and bandwidth capability.
Although these problems may be mitigated by employing expensive techniques, e.g., decreasing fiber length between optical nodes, such techniques may prohibitively increase costs to both subscribers and service providers. Accordingly, there exists a need for more reliable and accurate transmission of optical signals within a cable communications system. In particular, improved optical signal reliability and accuracy in reverse path transmissions is critically needed.